This patent document relates to detection of cosmic radiation and imaging based on imaging based on cosmic-ray produced charged particles.
Cosmic ray tomography is a technique which exploits the multiple Coulomb scattering of cosmic ray-produced charged particles (e.g., muons) to perform non-destructive inspection of the material without the use of artificial radiation. The earth is continuously bombarded by energetic stable particles, mostly protons, coming from deep space. These particles interact with atoms in the upper atmosphere to produce showers of particles that include many short-lived pions which decay producing longer-lived muons. Muons interact with matter primarily through the Coulomb force having no nuclear interaction and radiating much less readily than electrons. Such cosmic ray-produced particles slowly lose energy through electromagnetic interactions. Consequently, many of the cosmic ray-produced muons arrive at the earth's surface as highly penetrating charged radiation. The muon flux at sea level is about 1 muon per cm2 per minute. Also at sea level, there exists a flux of cosmic ray generated electrons, from delta ray production (electron knock-out), Bremsstrahlung or the decay of particles in cosmic ray induced showers. The electron flux at sea level is about 1 electron per cm2 per 3 minutes.
As a charged particle such as a muon moves through material, Coulomb scattering off of the charges of sub-atomic particles perturb its trajectory. The total deflection depends on several material properties, but the dominant effect is the atomic number, Z, of nuclei. The trajectories of charged particles (e.g., muons) are more strongly affected by materials that make good gamma ray shielding, such as lead and tungsten, and by special nuclear materials (SNMs), such as uranium and plutonium, than by materials that make up more ordinary objects such as water, plastic, aluminum and steel. Each charged particle (e.g., a muon) carries information about the objects that it has penetrated. The scattering of multiple charged particles (e.g., muons) can be measured and processed to probe the properties of these objects. A material with a high atomic number Z and a high density can be detected and identified when the material is located, inside low-Z and medium-Z matter.
Coulomb scattering from atomic nuclei in a matter results in a very large number of small angle deflections of charged particles as the transit the matter. A correlated distribution function can be used to approximately characterize the displacement and angle change of the trajectory that depends on the density and the atomic charge of the material. As an example, this distribution function can be approximated as a Gaussian distribution. The width of the distribution function is proportional to the inverse of the momentum of the particle and the square root of the real density of material measured in radiation lengths. The correlated distribution function of cosmic ray-produced charged particles (e.g., muons) can provide information on materials in the paths of the charged particles with no radiation dose above the earth's background and proper detection of such cosmic ray-produced charged particles (e.g., muons) can be implemented in a way that is especially sensitive to selected materials to be detected such as good radiation shielding materials.
A charged particle tomography detection system, e.g., a muon tomography detection system, can be configured to perform tomography of a target object under inspection based on scattering of charged particles by the target object and can be used as a portal monitor at various locations, such as border crossing points, ports, roadway checkpoints and other security checkpoints, for detecting certain targeted objects such as smuggled nuclear materials, nuclear and conventional weapons or other contraband. Charged particle tomography detector systems can be used jointly with or an alternative to other nuclear material detectors such as gamma or X ray detectors. Gamma and X ray detectors operate by directing Gamma and X ray radiation to a target and measuring penetrated Gamma and X ray radiation. Shielding of nuclear materials can reduce the count rates in the Gamma and X ray detectors and reduce the detection performance of Gamma and X ray detectors. Charged particle tomography detection systems can be configured to detect shielded nuclear materials and objects.